INTRODUCTION
Crossbreeding is the mating of animals of different breeds, and it is a breeding system that is practiced widely with cattle, sheep, pigs and poultry. The system has been used to:
- Produce commercial animals, particularly hybrid pigs and poultry;
- Produce new breeds such as the Brangus, Beefmaster etc.; and
- Produce polled breeds of cattle such as the polled Hereford.
OUTCROSSING
Outcrossing is the mating of unrelated animals of the same breed, and is used in pure breeding to offset the effect of inbreeding. It allows the introduction of genes from another herd. Another use of outcrossing is the ‘grading up’ of a herd. This is done by starting with a mixed bunch of non-pedigree animals, and by using pedigree bulls of one breed; the animals in the herd can achieve full pedigree status over a period of 4 to 5 generations. The objective is to gradually improve the quality of herds by the continuous use of good quality pedigree bulls.
The term ‘top crossing’ is used to mean the crossing of an inbred male with an unrelated female of the same breed.
2. THE EFFECTS OF CROSS BREEDING
GENETIC EFFECTS
Crossbreeding has the opposite effect to inbreeding, because it increases the number of heterozygous genes in the offspring produced by the cross.
| ABERDEEN ANGUS | X | RED SHORTHORN | ||||
| Homozygous for Black colour | BB | rr | Homozygous for red colour | |||
| F1 Generation | Br | Br | Br | Br | F1 Generation |
The offspring of this mating are all heterozygous for black colour because they carry a recessive gene for red colour. They will look black, but if they are mated together, they may produce black or red colour offspring.
Crossbred or outcrossed animals are much less likely to breed true than inbred animals.
PHENOTYPE EFFECTS
The first cross generation (the F1 generation) is usually very uniform in size and conformation, and this is a big advantage particularly in the production of meat animals such as beef, lambs, pigs and broiler chickens. However, if two crossbreds are mated, this uniformity is lost in the second generation, the F2 generation.
HYBRID VIGOUR OR HETEROSIS
This is the term given to the increased vigour which appears in the offspring of two unrelated animals that are mated together. This vigour affects the hardiness, viability, growth rate, fertility and milk yield of the crossbred animals and is most marked in the first cross, or F1 generation. Hybrid vigour has most effect on traits that have a low heritability. If two crossbred animals are mated together, much of the hybrid vigour is lost in the second generation, the F2 generation. However, if a crossbred dam is mated to a purebred sire, the hybrid vigour will be seen in the F2 generation.
Table 1: An Example of Hybrid Vigour Breeding
GENOTYPE EFFECTS
The genetic explanation for Hybrid Vigour or heterosis is that when two unrelated or distantly related animals are mated, a large number of fresh genetic combinations are produced. As many of these genetic combinations are heterozygous, the phenotype of the animal can be good, but the genotype will be poor and the crossbred animals will not ‘breed true’. This fact is the origin of the name heterosis for hybrid vigour.
The decline in vigour due to inbreeding, called inbreeding depression, indicates that many farm animals are carrying recessive genes that affect the vigour of the animal. These recessives can vary in effect from being lethal to slightly detrimental. Some of the most important production traits in farm animals are influenced by groups of genes, and inbred animals may be homozygous dominant or recessive for different genes within the group.
e.g.: First Animal AAbbccDD
This animal is homozygous dominant for AA and DD, and homozygous recessive for bb and cc.
Second Animal aaBBCCdd
This animal is homozygous dominant for BB and CC, and homozygous recessive for aa and dd.
If these two animals were mated together, the crossbred from the F1 generation would be:
Aa Bb Cc Dd
This animal has one dominant gene in each pair. Its phenotype would be good because of the dominant genes, but its genotype would be poor because each pair of genes is heterozygous with one dominant and one recessive gene.
When two animals pair off genetically like this, they are said to ‘nick’ well. The crossbred offspring have all the best traits of both parents together with hybrid vigour as a bonus. Poultry breeders usually breed several inbred lines of parent stock and mate them together until they find two lines that ‘nick’ together to give them a good hybrid.
Points to remember about hybrid vigour are:
- Traits that are expressed early in life, such as survival, general health and birth weights seem to be most affected by hybrid vigour. It has little influence on carcass quality but it does affect milk yields;
- Traits with a low heritability are influenced much more than those that are highly heritable.
- Traits that benefit most from heterosis are those that are most adversely affected by inbreeding; and
- The more distantly related the two parents the greater the effect of heterosis in the offspring. A greater effect can be expected by mating two different breeds (crossbreeding) than by mating two different lines within the same breed (out-breeding).
3. CROSSBREEDING IN SHEEP
Crossbreeding occurs widely in cattle, sheep, pigs and poultry to produce commercial animals. However, it should be remembered that there must be enough breeders of pure bred animals to maintain pure lines for crossing.
The classic example of crossbreeding is the stratification of sheep in the United Kingdom.
Table 2: Stratification of Sheep Example from the UK
The Cheviot is a breed of sheep adapted to the harsh climate and long winters in the North of England and Southern Scotland. They are hard, fairly fertile, and produce a coarse wool suitable for carpet manufacture, and they are slow to mature.
The Border Leicester is a much larger sheep than the Cheviot and the Pure Bred flocks are kept in the better climate of the lowland farms. This breed descended from the English Leicester, originally improved by Robert Bakewell in the 18th Century, and it is a good mutton sheep which also produces a heavy fleece of good quality wool. It is fairly fertile and slow maturing.
The Scotch Halfbred which is crossbred from the mating of the Border Leicester ram and the Cheviot ewe combines the best qualities of both the parent breeds together with hybrid vigour from the cross breeding. The ewes are larger than the Cheviot but smaller than the Border Leicester. They are hardy, good milkers and very fertile. Under good conditions of feeding and management, flocks of Halfbred ewes have achieved a lambing percentage of 200% e.g. each ewe in the flock has produced two lambs. They produce a fleece of good quality wool; they grow well, mature quickly and give good quality mutton.
The Suffolk is a large, slow maturing sheep with a light fleece of high quality wool. The crossbred lambs produced when the Suffolk ram is mated to the Scotch Halfbred ewes, are ideal butcher’s lambs; they produce very high quality meat, are fast to grow and mature, and are very uniform. Again, they combine the best qualities of both parents and their rapid growth and uniformity comes from the hybrid vigour produced by the second cross to an unrelated parent from a third breed.
This example of practical crossbreeding has been discussed in some detail because it shows how crossbreeding can be used to produce excellent commercial stock, with each generation being suited to its particular environment. The traits that are highly heritable are, conformation, meat and good quality, growth rate etc. these are passed on by the parent breeds. While hybrid vigour improves the milk yield, fertility and vigour of the Scotch Halfbred parent ewe. Crossbreeding also improved the uniformity of the final crop of fat lambs for slaughter.
The Dorper sheep is a good example of a new breed produced by crossbreeding and was bred from the following cross:
The Dorset Horn is a breed imported from the United Kingdom. It is a good meat producer with fine wool and is similar in type to the Suffolk breed. However, the main characteristic of the breed is that the ewes can be mated in both spring and autumn. This is unlike all other breeds of sheep from the United Kingdom whose ewes only come on heat in the autumn; their periods of oestrus are governed by decreasing hours of daylight.
The Blackhead Persian is a local breed of sheep. They are hardy and suited to hot, dry conditions and have a certain immunity to local parasites. However, they produce very poor quality meat and their coat is hair rather than wool. The ewes are not very fertile and, because of their harsh environment, they are poor milkers.
The Dorper was originally produced as a crossbred sheep, but the genes have been ‘fixed’ so that it is now a purebred and will breed pure. A Dorper ram mated to Dorper ewes will produce a fairly uniform crop of pure Dorper lambs. The breed combines the hardiness and adaptability to local conditions of the B.H.P. with the improved meat quality, fertility and milkiness of the Dorset Horn. Dorpers can be mated in the spring or the autumn. Their coat is a mixture of hair and wool and is of no value.
A common practice is to mate the Dorper ewe with the Mutton Merino Ram. This produces a uniform crop of lambs with good quality meat and wool, with all progeny being sold.
Below is an example of how a Dorper was bred the top two are the Dorset horn sheep (left) and the
Blackhead Persian (right) and the bottom picture shows the Dorper.
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